This invention relates, in general, to semiconductor device processing, and more particularly, to a method of preventing the degradation of dielectric integrity.
Dielectric rupture can occur when charge is injected into a semiconductor device structure comprising, a semiconductor substrate, a dielectric layer on the substrate and a floating semiconductor (or metal) layer on the dielectric layer. Because the semiconductor layer is floating its charge can not be easily discharged. Processes such as plasma etching, reactive ion etching (RIE), or other dry etching, and ion implantation can inject charge into a semiconductor device structure. If the charge gets large enough to induce high electric fields, the dielectric layer may rupture. This rupture can occur by hot electron damage. Rupture of a gate dielectric causes a degradation in the voltage/current characteristics of a semiconductor device.
A way to lower floating gate charge and reduce the severity of dielectric rupture during ion implantation, is to flood the wafer surface with electrons to neutralize the positive charge deposited onto the surface. This reduces the charging, but does not eliminate it because the flooding cannot be easily controlled for exact compensation. Also, flooding will tend to be over a large area and can result in a net negative gate charge which can also contribute to dielectric rupture. Even if electron flooding could neutralize the positive ions, secondary electrons would result in a net positive charge. In some ion implantation machines, electrons are mixed with the beam to partially neutralize it.
Another method proposed by J. C. Cheng, G. R. Tripp, J. A. Glaze, and J. R. Golin in a paper entitled, "Instrumentation for Neutralization of Wafer Charging," published in Nuclear Instruments and Methods in Physics Research B6 (1985), pp. 243-249, uses photon radiation (UV and X-ray) to increase the conductivity of the dielectric material to provide for charge neutralization. This method has two limitations: (1) when the photon radiation is used in the pulse mode, the power density (W/cm.sup.2) is limited by degradation of masking patterns on the wafer surface; (2) in the "dc" illumination mode, the oxide radiation damage limit may be reached. Another disadvantage is that UV and X-ray light are difficult to use.
By now it should be appreciated that it would be advantageous to provide a method of neutralizing the charge injected into MOS semiconductor device structures in order to prevent dielectric rupture.
Accordingly, it is an object of the present invention to provide a method for producing a large rate of free carriers, or electrons, in the semiconductor substrate to permit tunneling and eliminate bulk avalanche.
Another object of the present invention is to provide a method of reducing dielectric rupture.
A further object of the present invention is to neutralize the induced charge from plasma or ion implantation processing.
Yet another object of the present invention is to prevent hot electron damage of the substrate/dielectric interface during energetic ion beam processing.